88 THE BIOCHEMISTRY OF B VITAMINS 



is the Neurospora, which requires this single vitamin and no other. The 

 wild strains require biotin; they lack the enzyme (s) necessary for its 

 synthesis. The mutant strains in addition lack other specific enzymes due 

 to the destruction of specific individual genes. 



Of the work relating biotin to the activities of green plants, one of the 

 most significant bits is the demonstration that it acts as a growth sub- 

 stance in stimulating root production on etiolated cuttings of pea roots. 53 

 Stimulative effects of this sort suggest that biotin may not be produced 

 by green plants, or at least that the synthesis takes place in leaves rather 

 than roots, and that the roots may depend for at least part of their supply 

 on soil microorganisms. The stimulative effect of biotin on Rhizobia sug- 

 gests that the green plants may furnish biotin (synthesis taking place 

 probably in leaves) to the microorganism as an important factor in the 

 symbiotic relationship. The ability of some of these organisms to synthe- 

 size biotin at a slow rate has already been noted. No specific information 

 appears to be available regarding the relationship of biotin to mycorhizal 

 growth. In general, we may say that there appears to be no other B 

 vitamin for which production by microorganisms is as important as it 

 is in the case of biotin. 



Eakin and Eakin 54 have shown that pimelic acid stimulates the 

 synthesis of biotin by Aspergillus niger, and that this stimulation is 

 further enhanced by the presence of cysteine or cystine. Pimelic acid 

 and biotin have been found to be interchangeable as growth stimulants 

 for some organisms, so that it appears likely that pimelic acid is a pre- 

 cursor of biotin. There is some evidence to suggest that the synthesis 

 proceeds through desthiobiotin as a precursor (p. 468). 



Folic Acid, Inositol, Choline, p-Aminobenzoic Acid, "Vitamin Bi 2 " 



With respect to the biogenesis of other members of the B vitamin 

 family, hardly enough information is available to warrant more than a 

 very brief discussion. 



Folic acid is required by a considerable number of microorganisms, 

 but on the basis of available information it is produced by all those 

 bacteria which can grow on simple chemically defined media. 5 Its produc- 

 tion in the intestinal tracts of rats has been studied, 34 and numerous 

 demonstrations of deficiency have involved the prevention of intestinal 

 synthesis by sulfa drugs. Quantitative information in this field is rela- 

 tively unsatisfactory because of the difficulties of complete release of the 

 vitamin from its combined forms for assay (p. 40). The fact that deep 

 green leaves are an unusually rich source of folic acid, 55 as well as other 

 facts regarding its distribution, strongly suggest that this vitamin is 

 formed in green plants. 



